The present invention is related to an improved material for use as a performance additive in the conductive polymeric cathode of solid electrolytic capacitors.
Solid electrolytic capacitors are widely used throughout the electronics industry. In high voltage applications solid electrolytic capacitors with solid electrolytes, such as conductive polymer dispersions, give excellent high voltage performance compared to conductive polymer cathodes formed in-situ. These conductive polymer dispersions are prepared by polymerizing various conductive monomers in the presence of polyanions such as polystyrene sulphonic acid. Descriptions of these can be found in U.S. Pat. Nos. 5,300,575 and 7,990,684 and U.S. Pat. Publ. No. 2012057275 each of which is incorporated herein by reference. Conductive polymer dispersions suitable for solid electrolytic capacitors comprise polyethylene dioxythiophene:polystyrene sulfonic acid (PEDOT:PSSA), binders, additives, etc. Binders and additives provide the film forming, adhesion, and coating properties to these dispersions for applications in solid electrolytic capacitors.
U.S. Pat. No. 5,300,575, which is incorporated herein by reference, describes the use of polymeric binders for better adhesion and scratch resistance. Described therein is the use of polymeric binders soluble or suspended in water with the conductive polymer polyanion dispersions PEDOT:PSSA. Also described are polymer dispersions or lattices which are straight chain, branched or crosslinked polymers.
U.S. Pat. No. 6,987,663, which is incorporated by reference, describes several polymeric binders with PEDOT:PSSA dispersions. Binders listed include such materials as polyvinyl alcohols, polyvinyl pyrrolidones, polyvinyl chlorides, polyvinyl acetates, polyvinyl butyrates, polyacrylic acid esters, polyacrylic acid amides, polymethacrylic acid esters, polymethacrylic acid amides, polyacrylonitriles, styrene/acrylic acid esters, vinyl acetate/acrylic acid ester and ethylene/vinyl acetate copolymers, polybutadienes, polyisoprenes, polystyrenes, polyethers, polyesters, polycarbonates, polyurethanes, polyam ides, polyim ides, polysulphones, melamine formaldehyde resins, epoxide resins, silicone resins or celluloses. The polymeric organic binder described therein include those which may be produced by adding crosslinking agents, such as melamine compounds, masked isocyanates or functional silanes, such as 3-glycidoxypropyltrialkoxysilane, tetraethoxysilane and tetraethoxysilane hydrolysate or crosslinkable polymers, such as polyurethanes, polyacrylates or polyolefins and subsequent crosslinking. Crosslinking products of this type suitable as polymeric binders may also be formed, for example, by reacting the added crosslinking agents with the polymeric anions.
U.S. Pat. No. 5,415,893, which is incorporated herein by reference, describes the use of polymeric binders including polyester resin, polyurethane resins, acrylate resins, etc. in conductive polymer dispersions. U.S. Pat. No. 6,752,964, which is incorporated herein by reference, describes the use of a combination of conductive material, such as PEDOT:PSSA, and nonconductive polymeric materials such as poly(dienes), poly(alkenes), poly(acrylics), poly(methacrylics), poly(vinyl ethers), poly(vinyl thioethers), poly(vinyl alcohols), poly(vinyl ketones), poly(vinyl halides), poly(vinyl nitrites), poly(vinyl esters), poly(styrenes), poly(aryines), etc. poly(oxides), poly(carbonates), poly(esters), poly(anhydrides), poly(urethanes), poly (sulfonate), poly(siloxanes), poly (sulfides), poly(thioesters), poly(sulfones), poly(sulfonamides), poly(amides), poly(ureas), poly(phosphazens), poly(silanes), poly(silazanes), etc. poly(furantetracarboxylic acid diimides), poly(benzoxazoles), poly(oxadiazoles), poly(benzothiazinophenothiazines), poly(benzothiazoles), poly(pyrazinoquinoxalines), poly(pyromenitimides), poly(quinoxalines), poly(benzimidazoles), poly(oxidoles), poly(oxoisinodolines), poly(diaxoisoindoines), poly(triazines), poly(pyridzaines), poly(pioeraziness), poly(pyridinees), poly(pioeridiens), poly(triazoles), poly(pyrazoles), poly(pyrrolidines), poly(carboranes), poly(oxabicyclononanes), poly(diabenzofurans), poly(phthalides), poly(acetals), poly(anhydrides) and carbohydrates.
The material and process described in the above mentioned references have the disadvantage of higher equivalent series resistance (ESR) and unacceptable ESR shift. The higher viscosity of the polymer layer leads to a higher film thickness. The resulting capacitor has a high moisture sorption due to the hydrophilic nature of the binders and the higher film thickness. Current state of the art commercially available conductive polymer dispersions, suitable for solid electrolytic capacitors, use polyester sulphonic acid as a binder. These binders tend to absorb higher amounts of moisture due to the presence of sulphonic acid functionalities in the binder in addition to those in polyanion PSSA. Higher moisture absorption causes moisture outgassing when capacitors are exposed to the higher temperatures typically seen in board mount conditions. So a need exists for alternative materials which do not have additional sulphonic acid groups. Although binders promote adhesion promotion and coating coverage, a stable dispersion suitable for high performance capacitors needs rheology modifiers, wetting agents, defoaming agents, crosslinkers,etc. Formulators typically add these functional additives to fulfill a desired function. A consequence of the addition of insulative additives is that they could decrease the conductivity of the dispersion and could increase the ESR of the capacitor. It is thus desirable to identify a material or a performance additive which can provide the functions of binder, rheology modifiers, wetting agents, crosslinkers, adhesion promotion, etc. The prior art conductive dispersions do not meet these requirements